There has heretofore been known a process for producing a polyolefin by the use of a solid titanium-based catalyst and an organoaluminum compound. However, the polyolefin produced by the above-mentioned process generally has involved the problem that its transparency is poor when formed into films because of its being broad in the molecular weight distribution and compositional distribution.
On the other hand, there has recently been proposed a homogeneous catalyst having high activity and capable of producing a polymer with narrow molecular-weight distribution, which catalyst is exemplified by a catalyst composed of a metallocene compound of a transition metal and an aluminoxane. (Refer to Japanese Patent Application Laid-Open No. 19303/1983). Nevertheless, the aforesaid process for producing a polyolefin by the use of the above-mentioned homogeneous catalyst involves the problems that (1) the resultant polymer dissolves in a solvent or becomes gel, thereby making it difficult to separate the polymer from the solvent, (2) The polymer is likely to adhere to a reaction vessel and (3) a high-molecular weight polymer is difficult to produce.
There is proposed, as a process for producing a high-molecular weight polymer, a method in which a homogeneous catalyst composed of an oxygen-containing titanium compound and an aluminoxane is employed. (Refer to Japanese Patent Application Laid-Open No.3008/1988). However, the above-mentioned process suffers the disadvantage that the formed polymer adheres to a polymerization reactor, thus making it difficult to proceed with the reaction in a stable manner.
As a means for regulating the molecular weight distribution in the case of a homogeneous catalyst being used, there are proposed (1) a method in which at least two kinds of metallocene compounds are employed (Japanese Patent Application Laid-Open Nos. 35006/1985 and 35008/1985) and (2) a method in which polymerization is carried out at a specific temperature by the use of a hafnocene compound (Japanese Patent Application Laid-Open No. 75605/1990). However, any of the aforesaid methods suffers the disadvantage that the operation is troublesome or only a specific condition is utilizable for the method.
On the other hand, there is proposed a technique by using a solid substance which is obtained by allowing a homogeneous catalyst to be supported on an inorganic oxide carrier (Japanese Patent Application Laid-Open No. 108610/1985). Nevertheless, the above-proposed technique involves the problems of low activity of the catalyst per unit amount of the carrier and frequent occurrence of film gelling due to the carrier remaining in the polymer. There is also disclosed a catalyst composed of a non-metallocene transition metal compound and an aluminoxane that are supported on silica (Japanese Patent Application Laid-Open No. 503715/1989 through PCT). However, the above-mentioned catalyst involves the problem of low polymerization-activity in spite of a high-molecular weight of the polymer to be obtained.